Fertilization

All plants require certain chemical elements for normal growth. There are 16 elements, each of which is essential: the plant cannot complete its life cycle without them; and, no element can completely substitute for another.

Certain elements are permanently removed from soil over time. Some are removed each season, but ultimately returned in the form of fruit dropped and thinned, leaves and prunings. For example, 14-year old peaches producing 15 tons of fruit per acre have been shown to permanently remove: 68 pounds of nitrogen, 11 pounds of phosphorous, 86 pounds of potassium, 14 pounds of calcium, and 4 pounds of magnesium.

Fertilizers are classified into two broad groups: organic and non-organic (“conventional,” “chemical,” “commercial,” “inorganic,” etc). Organics are typically plant, animal or naturally occurring in origin. Non-organics (conventionals) are manufactured or formulated from many sources and processes. The plant does not recognize the source of ions absorbed.

Historically, both commercial and backyard orchards were established without pre-plant fertilization. Maintenance fertilizers, usually nitrogen (N), were applied after establishment. Greater reliance on soil tests prior to planting and use of organic supplements has accelerated in recent years. Many sources of fertilizers are readily available today along with composts and cover crops.

Fertilization should be considered as:

What can be done at planting and during the first year

Maintenance applications from second year on to full tree maturity

Nitrogen (N) is the nutrient most frequently found deficient in plants. The symptoms of nitrogen deficiency are:

Foliage lighter green in color

Poor growth

Smaller leaves

Potassium (K) is mobile and needed in many reactions in the plant. Except for sands, most mineral soils are originally high in total K. The symptoms of potassium deficiency are:

Restricted growth

Shoots die back in extreme cases

Leaves in the middle of the shoot area are affected, not basal leaves or shoot tips

Leaf margins and/or areas between veins may scorch

Interveinal chlorosis

Fruits tend to be smaller; fruit may shrivel when deficiency is severe

Leaf curling or cupping may be present

Phosphorous (P) deficiency is not often found except on red foothill and mountain soils. Available P comes from the breakdown of soil minerals or organic matter, and it is quite mobile in plants. Foliage of deciduous or evergreen trees deficient in P becomes dull and bronzed (even purplish) in late summer.

At Planting

A response to P (phosphorous) fertilization at planting is usually obtained in most foothill soils, especially red soils. P improves overall growth. When applied to the soil surface, P moves downward very slowly. Preplant incorporations are preferred for getting P down into the soil. A treatment at planting may last three to 10 years. One method is to place one of the following in the bottom of the planting hole, then cover with two to three inches of soil to prevent direct root contact:

* Some may be blended with backfill to obtain better placement in the hole without the risk of burning the roots.

Plant the tree. Then, spread an equal amount of one of the above on the soil surface around the tree.

First Year

In the first year, a very light application of nitrogen (N)—one-half to one ounce of actual N—is desirable on most soils. Do not make first year applications before six to eight inches of new growth. Split applications are safest. If split (one or two months apart), split the amounts listed below. Irrigation is required after application to move N to the root zone. Try one of the following:

1/8 to 1/4 lb. 33-0-0

3 lb. composted rabbit/poultry manure

7½ lb. steer to cow manure

N is usually applied near the edge of the tree canopy, but no closer than six to eight inches to the trunk of a small tree.

Second and Following Years

Research indicates that summer fertilizer applications (August to mid-September) are more efficient than late winter (traditional) applications.

Apply from one to four pounds of actual N per tree annually. Each of the following suggestions equal about one pound of actual N:

3 lb. 33-0-0

5 lb. 21-0-0

7 lb. 16-16-16

35 lb. of rabbit manure

70 lb. steer or cow manure

N is applied during the growing season followed by an irrigation. Many commercial orchards fertilize right after harvest, before the next-to-last or last irrigation. Water will move the fertilizer into the soil, roots will pick it up and store it, but there will not be enough time for the fertilizer to encourage undesirable new growth that late in the season. (Excess N fertilization may promote bitter pit in apples.) On red soils, P may also need to be applied to mature trees.

Actively transpirating leaves are needed for N uptake, so N should not be applied in the very late fall, winter, nor early spring. Be advised that excess water will leach the N fertilizer. Nitrates, the form of N trees uptake more readily, are very mobile and easy to leach beyond the root zone.

Many fertilizer materials can be found in the marketplace. Composts and green manure crops are good practices to bring into the picture of tree nutrition and healthy soils.

The table below gives the amount of N that should be applied per tree per year using ammonium sulfate (21-0-0) as an example. This amount is adjusted according to tree age/season.

potassium, boron, zinc, and magnesium deficiencies are occasionally seen on foothill tree crops. Remember: fertilizer suggestions or routines are no substitute for personally observing tree growth (amount of shoot growth and annual wood produced) and health (foliage color, fruit production and quality, etc).